1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device in which an image is displayed by reflecting light incident from the outside, and to a method for manufacturing the device.
2. Description of the Related Art
Recently, the application of a liquid crystal display device to word processors, lap top personal computers, pocket televisions and others is rapidly expanding. In particular, among liquid crystal display devices, a reflection type liquid crystal display device in which an image is displayed by reflecting light incident from the outside is highly attractive because it can be constructed in a reduced thickness and weight, and the amount of consumed electric power is low due to no backlight.
Conventionally, in a reflection type liquid crystal display device, the TN (twisted nematic) method or the STN (super-twisted nematic) method has been employed. In these methods, however, about half of a light intensity of natural light can not be utilized for displaying due to a polarizer, which causes a problem that the display is relatively dark.
To solve the problem, a display mode has been proposed in which no polarizer is used to effectively utilize natural light. An example of such a display mode is the phase transition type guest-host method (D. L. White and G. N. Taylor: J. Appl. Phys. 45 4718, 1974). In this mode, the cholesteric-nematic phase transition phenomenon due to an electric field is utilized. Also, a reflection type multicolor display is proposed, in which a micro color filter is additionally utilized in the method. (Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29/2, 157, 1988).
In order to obtain a brighter display in such a mode without requiring a polarizer, it is necessary to increase the intensity of light scattering in a direction perpendicular to the display screen, with respect to light incident at any angle. For this purpose, it is necessary to produce a reflector having an optimum reflective characteristic. The above publication discloses a reflector which is produced by toughening the surface of a substrate made of glass or the like with an abrasive, varying the time of etching with hydrofluoric acid to control the surface roughness, and forming a silver thin film on the rough surface.
In the reflector disclosed in the publication, the irregularities can not be uniform in shape because the irregularities are formed by grinding the glass substrate with an abrasive. The reflector has another problem that the reproducibility of the shape of the irregularities is poor. In the case of using such a glass substrate, consequently, it is impossible to provide a reflection type liquid crystal display with excellent reproducibility and an optimum reflective characteristic.
FIG. 15 is a plan view showing an insulating substrate 2 having a thin film transistor (hereinafter abbreviated as "TFT") 1 which is a switching element used in an active matrix system, and FIG. 16 is a sectional view showing the insulating substrate 2 as taken along line X1--X1 of FIG. 15. Plural gate bus wirings 3 made of chromium, tantalum or the like are disposed in parallel on the insulating substrate 2 made of glass or the like, and a gate electrode 4 is branched off from the gate bus wiring 3. The gate bus wiring 3 functions as a scanning signal line.
A gate insulating film 5 made of a silicon nitride (SiN.sub.X), a silicon oxide (SiO.sub.X) or the like is formed on the entire surface of the insulating substrate 2 to cover the gate electrode 4. On a gate insulating film 5 above the gate electrode 4, formed is a semiconductor layer 6 made of amorphous silicon (hereinafter abbreviated as "a-Si"), polycrystalline silicon, CdSe or the like. A source electrode 7 made of titanium, molybdenum, aluminum or the like is superposed on one end of the semiconductor layer 6. A drain electrode 8 made of titanium, molybdenum, aluminum or the like in the same manner as the source electrode 7 is superposed on the other end of the semiconductor layer 6. A picture element electrode 9 made of, for example, ITO (indium tin oxide) is superposed on one end of the drain electrode 8 that is opposite to the end on which the semiconductor layer 6 is superposed.
As shown in FIG. 15, a source bus wiring 10 is connected to the source electrode 7. The source bus wiring 10 cross the gate bus wiring 3 through the gate insulating film 5 therebetween, and functions as an image signal line. The source bus wiring 10 also is made of the same metal as that of the source electrode 7. A TFT 1, which functions as a switching element, comprises the gate electrode 4, the gate insulating film 5, the semiconductor layer 6, the source electrode 7, and the drain electrode 8.
When the insulating substrate 2 having the TFT 1 shown in FIGS. 15 and 16 is to be applied to a reflection type liquid crystal display device, the picture element electrode 9 is made of a metal having light reflectivity such as aluminum or silver, to be employed as a reflector.
Employing the picture element electrode 9 as a reflector has advantages that the thickness can be reduced and degrading of display due to parallax is small as compared with the case where a reflector is separately disposed on the side of the insulating substrate 2 opposite to the picture element electrode 9.
In order to further improve the reflective characteristic, irregularities are formed on the surface of the picture element electrode 9 superposed on the gate insulating film 5 as a consequence of forming the irregularities on the surface of the gate insulating film 5. Usually, however, it is difficult to uniformly form irregularities on the surface of the gate insulating film 5 made of an inorganic substance.
As described above, a semiconductor layer 6 is made of a-Si, polycrystalline silicon, CdSe or the like. The semiconductor layer 6 made of a-Si, in particular, has an advantage that it can be uniformly formed at a low temperature into that of a large area, compared with the other materials. However, it has a characteristic that light photoelectric current is generated. To be more precise, though the band gap of Si is 1.1 eV, holes and electrons are hardly generated in Si because it is an indirect transition type material. On the other hand, a-Si has a high probability of generation of holes and electrons due to levels in band gaps of a-Si, and therefore holes and electrons are easily generated by virtue of light. Therefore, when light enters the TFT 1, therefore, the TFT 1 is driven to operate by virtue of the light, so that it cannot properly function as a switching element. This causes inconvenient phenomena such as that an undesirable image signal is applied to the picture element electrode 9, thereby producing a disadvantage that the display quality is impaired. In a substrate opposing the insulating substrate 2, therefore, it is usual that a black matrix which is light shielding means is formed in the portion of the substrate which faces the TFT 1. However, it is difficult to completely shield light via the black matrix because of such problems that the accurate positioning is required, and that obliquely incoming light cannot be shielded.
As shown in FIGS. 15 and 16, in order to prevent the picture element electrode 9 and the source bus wirings 10 from being electrically connected to each other, they are formed so as to be separated from each other by a gap 9a. When the source electrode 7 and the drain electrode 8 are electrically connected to each other, moreover, the TFT 1 cannot function as a witching element. Therefore, the picture element electrode 9 cannot be formed on the TFT 1, resulting in that the area of the picture element electrode 9 is small. This causes a problem in that the luminace is low and therefore the display quality is impaired.